1. Field of the Invention
The present invention relates generally to synthetic aperture radar (SAR) and, more particularly, to an apparatus and method of employing a plurality of SARs mounted on an aircraft or moving platform for imaging moving targets and determining their velocity components.
2. Description of the Prior Art
Conventional SAR (synthetic aperture radar) is used for the remote sensing of earth resources in such fields as hydrology, agriculture, forestry, and geology, just to name a few; and for the mapping of rural areas and urban centers. In addition, conventional SAR is used to detect, image, and locate targets within the scene. All of these applications utilize signal processing schemes which require that objects within the scene remain stationary.
The imaging of stationary objects or targets within the scene is accomplished with a synthetic narrow beam antenna, which is created when an airborne SAR flies a long straight line path, called a synthetic aperture. Over this path, the SAR periodically transmits a pulsed frequency modulated signal towards the ground, in the direction transverse to the flight path and at some depression angle with respect to the horizontal, and recovers the back scattered signal from the target using quadrature mixers. The mixer outputs are then combined to form a complex two dimensional IF (intermediate frequency) output signal. This signal is comprised of two orthogonal fluctuations. One fluctuation, called the range fluctuation, occurs in the fast time dimension, and is due to the round trip delay time from the SAR to the target and back. Fast time is the time axis associated with transmitting and receiving a pulse at a downtrack position. The other fluctuation, called the doppler fluctuation, occurs in the downtrack position dimension, and is due to the relative motion of the target with respect to the SAR. Range compression is accomplished by taking the Fourier transform of the IF output signal with respect to fast time. The resulting doppler track is then compressed in the downtrack position dimension by correlating it with the doppler chirp reference for a stationary point target. The image produced is spatially resolved and is centered at the proper range and downtrack position.
When targets are in motion the conventional SAR's ability to perform is either severely degraded or lost. For example, a loss in signal strength, a degradation in image resolution, and an offset in target location can result. Consequently, conventional SAR-based ground surveillance systems are not very effective for monitoring targets in motion.
A literature search was conducted in order to obtain works in the field of moving target imaging. Many excellent papers on conventional SAR were uncovered. The best and most recent works, which examine the problem of imaging moving targets and propose a solution, are two very similar papers by Freeman entitled, "Simple MTI Using Synthetic Aperture Radar," Proc. of IGARSS 1984 Symposium, ESA SP-215, pp. 65-70, and Freeman et al. entitled, "Synthetic Aperture Radar (SAR) Images of Moving Targets," GEC Journal of Research, Vol. 5, No. 2, pp. 106-115. In these papers, the SAR's pulse repetition frequency is made significantly greater than the doppler bandwidth (or clutter band) associated with a conventional SAR, so that moving target returns with doppler frequencies outside the clutter band can be recovered. A bank of doppler filters, each of bandwidth equal to the clutter band, subdivide the doppler frequency domain into non-overlapping doppler bands. Notably, each doppler band corresponds to a different radial velocity band of target motions. The doppler filters are used to sort moving targets in the SAR return according to radial velocity. For a given filter the output is undersampled, so that the filter's band center is aliased onto zero doppler frequency, and the resulting samples are compressed in the downtrack position dimension using the conventional SAR reference. The resulting SAR image spatially resolves moving targets with a downtrack positional uncertainty given by .+-.R.sub.0 .theta./2 and a radial velocity uncertainty given by .+-.V.sub.p .theta./2. Here R.sub.0 is the target broadside range, .theta. is the antenna 3 dB beamwidth, and V.sub.p is the platform velocity. These uncertainties can be quite large. Further, some degradation in spatial resolution is expected, because target downtrack velocity is not compensated for.